life, species, organism.... all are leaky abstractions.

[There's a "species" that spread around the Andes and eventually reached itself on the other side---and could no longer breeed with "itself". Genes could still in theory travel all the way around and back to the discontinuous point, however.]

edit: http://i.stack.imgur.com/4VsFW.png will make this less confusing, and quite unintentionally :).

Having thought about this a lot, the best definition of life I've been able to come up with is as follows:

An object is considered to be alive if it consumes energy to create or maintain its own order (in opposition to the general effect of the second law of thermodynamics in the wider system in which it finds itself).

Although this doesn't exclude crystals..! :/

Bacteria can do things on their own (say, swim in water). Viruses can't do anything without a host's reproduction engine (like a program which can't do anything without a computer to run on). Prions are misshapen proteins which tend to damage other proteins such that the result is the same misshapen, and thus duplicating, form.

I think what was meant is that on the grand scale of "aliveness" viruses are more alive than prions. Viruses have DNA, they actively seek out host cells to hijack so they can reproduce, etc. Prions are much simpler and aren't alive at all. They're just accidentally self-reproducing patterns, basically. They don't have DNA, they don't make any specific effort to reproduce, they just "happen" from weirdly broken matter bumping together.

Viruses don't make any specific effort to reproduce, they too are just accidentally self-reproducing patterns.